A conventional cardiac pacemaker is an implantable battery-powered electronic device that responds to sensed cardiac events and elapsed time intervals by changing its functional states so as to properly interpret sensed data and deliver pacing pulses to the heart at appropriate times. The pacing pulses are delivered through a lead made up of electrodes on a catheter or wire that connects the pacemaker to the heart. Modern pacemakers are typically programmable so that they can operate in any mode which the physical configuration of the device will allow. Such modes define which heart chambers are paced, which chambers are sensed, and the response of the pacemaker to a sensed P wave or R wave. A three-letter code is used to designate a pacing mode where the first letter refers to the paced chamber(s), the second letter refers to the sensed chamber(s), and the third letter refers to the response. Additional sensing of physiological data allows some pacemakers to change the rate at which they pace the heart in accordance with some parameter correlated to metabolic demand. Such pacemakers, which are the primary subject of the present invention, are called rate-adaptive pacemakers.
The most common condition for which pacemakers are used is the treatment of bradycardia. Permanent pacing for bradycardia is indicated in patients with symptomatic bradycardia of any type as long as it is likely to be permanent or recurrent and is not associated with a transient condition from which the patient may recover. Atrio-ventricular conduction defects (i.e., AV block) that are fixed or intermittent and sick sinus syndrome represent the most common indications for permanent pacing. In chronotropically competent patients in need of ventricular pacing, atrial triggered modes such as DDD or VDD are desirable because they allow the pacing to track the physiologically normal atrial rhythm, which causes cardiac output to be responsive to the metabolic needs of the body. Atrial triggering modes are contraindicated, however, in patients prone to atrial fibrillation or flutter or in whom a reliable atrial sense cannot be obtained. In the former case, the ventricles will be paced at too high a rate. Failing to sense an atrial P wave, on the other hand, results in a loss of atrial tracking which can lead to negative hemodynamic effects because the pacemaker then reverts to its minimum ventricular pacing rate. In pacemaker patients who are chronotropically incompetent (e.g., sinus node dysfunction) or in whom atrial-triggered modes such as DDD and VDD are contraindicated, the heart rate is determined solely by the pacemaker in the absence of intrinsic cardiac activity. That heart rate is determined by the programmed escape intervals of the pacemaker and is referred to as the lower rate limit or LRL.
Pacing the heart at a fixed rate as determined by the LRL setting of the pacemaker, however, does not allow the heart rate to increase with increased metabolic demand. Cardiac output is determined by two factors, the stroke volume and heart rate, with the latter being the primary determinant. Although stroke volume can be increased during exercise, the resulting increase in cardiac output is usually not sufficient to meet the body's metabolic needs unless the heart rate is also increased. If the heart is paced at a constant rate, as for example by a VVI pacemaker, severe limitations are imposed upon the patient with respect to lifestyle and activities. It is to overcome these limitations and improve the quality of life of such patients that rate-adaptive pacemakers have been developed. Rate-adaptive pacemakers operate so as to vary the lowest rate at which the heart is allowed to beat in accordance with one or more physiological parameters related to metabolic demand.
The body's normal regulatory mechanisms act so as to increase cardiac output when the metabolic rate is increased due to an increased exertion level in order to transport more oxygen and remove more waste products. One way to control the rate of a pacemaker, therefore, is to measure the metabolic rate of the body and vary the pacing rate in accordance with the measurement. Metabolic rate can effectively be directly measured by, for example, sensing blood pH or blood oxygen saturation. Practical problems with implementing pacemakers controlled by such direct measurements, however, have led to the development of pacemakers that are rate-controlled in accordance with physiological variables that are indirectly reflective of the body's metabolic rate such as body temperature, ventilation rate, or minute ventilation. Minute ventilation varies almost linearly with aerobic oxygen consumption during exercise up to the anaerobic threshold and is the physiological variable that is most commonly used in rate-adaptive pacemakers to reflect the exertion level of the patient.
An even more indirect indication of metabolic rate is provided by the measurement of body activity or motion. Body activity is correlated with metabolic demand because such activity requires energy expenditure and hence oxygen consumption. An activity-sensing pacemaker uses a piezoelectric sensor or accelerometer inside the pacemaker case that responds to vibrations or accelerations by producing electrical signals proportional to the patient's level of physical activity.
In such rate-adaptive pacemakers that vary the pacing rate in accordance with a measured exertion level, the control system is generally implemented as an open-loop controller that maps a particular exertion level to one particular target heart rate. The mapping is accomplished by a rate-response curve which is typically a linear function (i.e., a straight line), but could also be some non-linear function as well such as a dual-slope curve or exponential curve. The rate-response curve is then defined with minimum and maximum target heart rates. A minimum target heart rate for a patient can be ascertained clinically as a heart rate adequate to sustain the patient at rest, while a maximum allowable target heart rate is defined with a formula that depends on the patient's age. The rate-response curve then maps a resting exertion level to the minimum heart rate and maps the maximum exertion level attainable by the patient, termed the maximum exercise capacity, to the maximum allowable heart rate. The responsiveness of the control system, defined as how the target heart rate changes with a given change in exertion level, depends upon the slope of the rate-response curve (or slopes in the case of a dual-slope curve) which is dictated by the defined maximum exercise capacity. If the maximum exercise capacity is incorrectly defined, the pacemaker's responsiveness will not be set to an appropriate level. An under-responsive pacemaker will unnecessarily limit exercise duration and intensity in the patient because the heart rate will not increase enough to match metabolic demand, while an over-responsive pacemaker can lead to palpitations and patient discomfort.
In order to define a patient's maximum exercise capacity, exercise testing can be performed to determine the maximum exertion level that the patient is capable of attaining. The pacemaker can then be programmed with that value with adjustments made during follow-up clinical visits. Exercise testing may not always be practical, however, and a patient's maximum exercise capacity can change over time due to, e.g., physical conditioning, illness, or recovery from illness, which increases the need for follow-up visits. Algorithms have therefore been developed that attempt to adjust the responsiveness of rate-adaptive pacemakers automatically in accordance with exertion level measurements made as the patient goes about ordinary activity. Determining a patient's maximum exercise capacity from periodic exertion level measurements, however, is problematical since it is not known how close to the true maximum a periodic maximum exertion level is.